The present invention relates to a device and a method for continuous pressing of a plastically deformable blank, for example made of a metal, into a three-dimensional section with a predetermined cross-sectional area, comprising a fixed die with an opening formed in the die, through which the plastically deformable blank is intended to be pressed, and at least one rotary die arranged, adjacent to the opening, around an axis extending transversely of the press direction, the die having one or more recesses in its peripheral surface for forming the blank into a three-dimensional section with transverse sectional parts during the rotation of the rotary die.
In continuous pressing of a plastically deformable blank, for example a heated metal such as aluminium, so-called extrusion, the blank passes an opening with a desired cross-sectional area, thereby forming a section whose longitudinal cross-section is constant. There is a great need for continuous manufacture of sections with transverse sectional parts, such as racks, hollow sections, etc.
International Patent Specification WO97/12745 discloses a method and a device invented by the present inventor, which aim at allowing extrusion of sections with sectional parts protruding transversely of the section. According to this publication, a rotary die is arranged to constitute part of the opening through which the blank is pressed. As the cross-sectional area of the blank is being reduced, the rotating die simultaneously forms it. The rotary die can be designed to produce transverse bars in the section, or to form a raised or embedded company name in the section.
The difference compared to various types of die stamping with rotating elements is to be noted, illustrated for example in DE 42101746, where only a very limited forming of the blank takes place. When shaping according to the above technique, as referred to by the present invention, the rotating die forms part of the actual extrusion process.
The application of this technique in existing, largely standardised, press facilities such as hydraulic pressing plants, screw extruders, conform extrusion machines, etc, was previously impossible. Facilities of said type usually comprise a tool arrangement of the type shown in FIG. 2, with a support 5 for a substantially cylindrical tool 3 comprising a fixed die 1. There is not much space around this tool, and the forces generated during the pressing are very strong.
Furthermore, it is very important that the number of production stoppages be reduced, since the cost of unexploited machine capacity is very high. It is, therefore, desirable that tools can be changed rapidly according to pressing needs.
Since Patent Specification WO97/12745 was published, the need for sections with a cross-sectional area that varies longitudinally has arisen, i.e. a section having not only transverse sectional parts such as bars, but also a varying cross-section or material thickness along the continuous section.
The object of the present invention is to provide a device for pressing three-dimensional sections, which is easy to apply to moulds according to prior art, with no need for major adjustments.
This object is achieved by means of a device and a method of the type described by way of introduction, wherein said rotary die is arranged immediately downstream of said opening, whereby the blank is reduced when passing through said opening (11) to substantially the predetermined cross-sectional area, and then formed when passing said rotary die, thereby determining the final shape of the three-dimensional section.
Unlike prior art, the area of the blank is thus reduced substantially down to its final cross-sectional area upstream of the rotating die, whereby the forces acting on the rotating die can be minimised. This results in manageable bearing forces, which allows the bearings of the rotary die to be contained in the fixed die. The expression xe2x80x9csubstantially down toxe2x80x9d means primarily down to between 100% and 130% of the final pre-determined cross-sectional area.
The blank meets with the rotating die radially within its average radius. In this way, some area reduction still takes place at the rotating die, and thus a certain acceleration of the blank occurs during this passage while at the same time the material fills cavities in the rotating die.
The expression xe2x80x9cimmediately downstream ofxe2x80x9d means that the rotary die is located so close to the opening that the pressure of the pressing is used in the shaping done by the rotating die. If the distance is too long, for example several times the across corner dimension of the section, the blank will self-lock adjacent to the rotating die because of the friction caused upstream against the supporting surfaces when the rotating die is in a pressing phase.
The rotary die is preferably mounted in bearings in a transverse cavity formed next to the opening, thereby being rotatable around an axis extending transversely of the pressing direction.
This design of the fixed die allows a space-efficient location of the rotary die within the machine. Furthermore, this construction means that the rotary die is easily accessible, since it is relatively easy to loosen and remove the tool in a normal compression moulding machine. Thus, the device can be designed so as to be compatible with conventional extruding machines in order to allow rapid changing of tools without the need for expensive production stoppages.
By forming a cavity in the fixed die, the space is used as much as is possible, and, in addition, a smaller amount of toughened material is needed for the fixed die, which reduces the cost.
The rotary die is preferably mounted in bearings with a certain axial play. This play allows some thermal expansion of the rotating die without causing any jamming.
The rotary die may be fixedly arranged on a shaft mounted in bearings in the cavity, the shaft having a limited axial play. Thus, owing to this construction the shaft is axially guided by the rotary die. Since the shaft and its bearings are arranged in the fixed die, this constitutes a unit in which the rotary die is arranged, the unit being easily replaceable. Moreover, the shaft may be relatively short, which results in a favourable load take-up capacity and less load on the bearings.
A shaft portion extending through the rotary die can be made of a material with a higher thermal expansion coefficient than the rotary die, so that said shaft portion, when the rotary die and the shaft are heated during pressing, expands more than the rotary die, which is thereby secured to the shaft. By using this technique to secure the rotary die, the need for securing elements in the shaft and the die is eliminated.
The opening preferably comprises a recess in the fixed die on the upstream side, which is intended to cause a first cross-sectional reduction of the material, the recess being substantially formed on the side of the opening opposite to the cavity. By forming the recess in this way, there is less stress on the fixed die at the cavity in which the rotary die is arranged. In a traditional type of tool, where the corresponding recess usually is symmetrical, the material around the cavity may become too thin.
According to a second aspect of the invention, the device further comprises means for varying the cross-sectional area immediately upstream of the rotary die. In other words, the fixed die is arranged to have an opening with a variable cross section. Thus, the amount of material pressed against the rotary die may be varied, suitably according to the shape of the rotary die.
The peripheral surface of the rotary die may, for example, present sectors with varying radius, which permits pressing of sections with varying cross-sectional area.
By xe2x80x9cperipheral surfacexe2x80x9d is here meant the normally circular-cylindrical surface in which different kinds of recesses or protrusions have been made for forming the sections, for example the surface that is made up by the pitch radius of a gear wheel. The fact that the radius of the peripheral surface varies could mean, for example, an oval-shaped die (such as a gear wheel with varying pitch radius), or that the shaft is arranged in connection with the rotary die slightly offset relative to the centre of the die. This would result in a section, whose continuous material thickness would vary cyclically, which is desirable when manufacturing a beam with varying strength.
The means for varying the cross-sectional area are suitably synchronised with the rotary die and may consist of supporting surfaces moveable transversely of the pressing direction.
According to a third aspect of the invention, the rotary die is arranged to be lockable in a predetermined position. Thus, the rotary, moveable die may be locked, and thereby essentially converted into a fixed die. Pressing may now take place, either by passing one rotary-die or by passing one or more fixed dies, which offers improved possibilities of varying the pressed sections.
The rotary die may suitably have smooth sectors, which in the locked position face the blank, so that, in this position, the blank passes the locked die for forming a smooth sectional segment. By orienting a smooth sector so that it faces the blank when locking the rotary die, the forces acting on the rotary die in the locked position are minimised. Locking the rotary die in a position where recesses or protrusions are oriented so that they face the blank would in fact require a great locking force and would, in addition, mean a risk of loose pieces forming in the cavities of the die during pressing.